Hey Crispy
Well my thoughts at the moment - always changing- go something like this. 18-20' w x 30-40' long.
I'd use it mostly to extend the seasons as you mentioned, although some of the cold weather crops might hang in a bit longer.
I will definatly devide it into at least 2 sections using plastic or plywood and put most of my assets into the part closest to the house to hopefully keep that part a little more comfey. It might all have a film covering, or I might use twinwall in the "high-end" section and possibly harvest some of the warm air for heating the house and do my beer drinking there.
Snow will be an issue. I see a lot of film covered greenhouses on farms, that have a more pointed profile with steeper walls, for this reason. I also keep thinking about making the top section from corrugated plastic panels, and using film for the sides to help with the snow. It might be possible to make some of the panels so they can be raised as a vent to let excess heat out.
At some point I will be adding a Rocket Stove to keep seed beds warm, and put some BTUs into the mass.
That's it for now. I hope you read through the greenhouse post below - lots of good info.
Pete